1. Field of the Invention
The present invention relates to an epoxy resin type composition for stiffening a vehicle body and a method for stiffening a vehicle body, and, in particular, to an epoxy resin type composition for stiffening a vehicle body for improving a rigidity of a box type structural member with a closed section constituting a part of a vehicle body and a method for stiffening a vehicle body by use of the same.
2. Description of the Related Art
Although a conventional vehicle body skeleton having a box type structural member with a closed section is produced by giving various sectional shapes to such a closed sectional structure, there is demand for reduction of its body weight from social demand for reduction of mileage from a viewpoint of the tendency of exhausting fossil fuels and for reduction of exhaust gas from a viewpoint of air and environmental protection.
Meanwhile, a vehicle body is stiffened due to demand for noise and vibration performance, safety in collision and safety for vehicle driving control, whereas there is the tendency that the body weight of a vehicle is increasing year by year. The method for stiffening a vehicle body generally uses a metallic stiffening material in a portion or portions considered effective for the vehicle body. On the other hand, a body structure of a vehicle whose body skeleton is stiffened by charging a foamed body of a hard urethane resin into the inside of a closed section of the vehicle body skeleton is well-known.
Charging of this foamed body exerts, a high inhibitory effect on buckling of the wall of the box type structural member of the vehicle body skeleton to significantly increase the rigidity of the box type structural member. Accordingly, as compared with the method for stiffening the vehicle body by metallic stiffening materials, there is the advantage that rigidity can be improved without significantly increasing the body weight.
Besides the urethane resin, resin foaming type fillers used in such foamed body charging include olefin resin type foaming fillers (Nippon Shihka, trade name: Shihka Lastmer 240), epoxy resin type foaming fillers (Iida Sangyo, trade name: OROTEX 815) etc., and any of these fillers are charged by being foamed in the step of coating line of a vehicle body.
However, in the method for stiffening the vehicle body by the foamed body of the urethane resin, there occurs considerable leakage of the urethane resin through small holes in the box type structural member and the portions of the box type structural member to be attached to each other, at the time of introduction of a stock solution of the urethane resin into the box type structural member of the vehicle body, or at the time of foaming in the step of the coating line of the vehicle body. Accordingly, measures to prevent such leakage should be taken, so it is considered difficult to apply this method to an actual mass-production line for vehicles.
Further, the foaming step of the actual mass-production line use in recent years is the step of using water in place of the step of using Freon from a viewpoint of improvement of working environments. However, it is difficult to achieve foaming homogeneity by the foaming step of using the water as compared with the foaming step of using the Freon.
Furthermore, it is a difficult operation for workers to introduce with an injection machine the stock solution of the urethane resin into vehicle bodies carried by a conveyer in a process of producing vehicles.
In addition, low-molecular-weight polyethylene wax etc. are used in an olefin resin as base in the olefin resin type foaming fillers. Accordingly, the rigidity of the material is not sufficient, and even charged into a box type structural member, its effect of improving rigidity is not sufficient.
With respect to epoxy resin type foaming fillers, it should be taken into consideration that their toughness is insufficient. This is because the input of loading onto a vehicle body is not necessarily static and the running vehicle body will undergo dynamic loading or even impact loading in some cases on a bumpy road, so impact resistance durable to such dynamic loading and impact loading, that is, toughness is required.
The method for improving this impact resistance is divided roughly into a method of improving the chemical structure itself of the epoxy resin and a method of adding a separately prepared impact-resistant improver to the epoxy rein. However, an epoxy resin satisfying impact resistance sufficiently has never been attained by using only the former method. On the other hand, the latter method includes the following conventional methods: (a) a method of adding a soluble elastomer monomer to a green epoxy resin and polymerizing them, (b) a method of adding a compatible elastomer polymer to a green epoxy resin, and (c) a method of dispersing an impact resistance-improving polymer in a green epoxy resin, etc.
Among these, the above method (a) is known as giving IPN (Inter-Penetrating Network). However, this prior method has the problem that along with reduction in the softening point of its product, its mechanical properties are not constant. For the above method (b), there are many proposals on rubber modification by adding elastomer components such as butadiene-acrylonitrile copolymer rubber (CTBN or ATBN) having a carboxyl group or amino group at the terminals, and some proposals are put to practical use. However, it cannot be said that the product obtained in this method is completely satisfactory in respect of impact resistance and toughness for use in stiffening a vehicle body to improve its rigidity. Further, in the above method (c), many resistance impact improvers including polyamide type resin have been proposed, but there is a problem with compatibility with the epoxy resin, and it is also difficult to regulate its viscosity at the time of foaming. Further, if a polymer with low glass transition temperature is mixed, storage stability may be deteriorated.
More specifically, if the viscosity of the resin used as foaming fillers is rapidly decreased in a foaming temperature range, generated gas is difficult to maintain in the resin, so foaming state is broken, thus making formation of its foamed body difficult. That is, it is necessary to regulate temperature dependency of resin viscosity in the foaming temperature range in order that so-called foamed cells are stably present in the resulting foamed body. For this, elastomer etc. with high compatibility with the epoxy resin are added to regulate the temperature dependency of viscosity or to regulate the temperature dependency of viscosity by cross-linkage.
However, the rigidity inherent in the epoxy composition will be lowered upon regulation of viscosity by adding the elastomer. Alternatively, if the viscosity is regulated by the cross-linkage accompanying chemical reaction, its cross-linkage density is subject to the reaction conditions and cannot be strictly regulated. Accordingly, there is the possibility that the viscosity is so high that the foaming is insufficient and the product cannot be sufficiently filled in a box type structural member of a vehicle body.
In addition, the above-described conventional foaming fillers are generally formed into a sheet, which, by use of its adhesion or by providing an adhesive layer, adheres to a constituent panel of the box type structural member. Thereafter, the vehicle body is assembled and passed through a chamber of an electrodeposition such as electrostatic coating in many cases. In such attachment of the sheet to the vehicle body, however, the electrodeposition coating can not cover the surface to which the sheet of the foaming fillers was attached. For example, if such a sheet is attached to a vertical panel of the box type structural member, the sheet is moved downward at the time of baking of the electrodeposition coating, and the raw surface of the constituent panel of the box type structural member is exposed, so there may be the possibility that anti-corrosive performance is lowered.